Dissolution of metals utilizing an aqueous H2 SO4 -H2 O2 -3-sulfopropyldithiocarbamate etchant

ABSTRACT

Improved metal dissolution rates are obtained when using a solution containing sulfuric acid, hydrogen peroxide and a catalytic amount of 3-sulfopropyldithiocarbamate sodium salt having the general formula ##STR1## wherein R can be a C 1  -C 4  alkyl group, phenyl, a C 7  -C 10  alkaryl group or a C 7  -C 10  aralkyl group.

The present invention relates to the dissolution of metals in an aqueousbath containing sulfuric acid and hydrogen peroxide, and in particularto a novel bath composition capable of effecting the dissolution at highrates. In one specific aspect the invention is concerned with etching ofcopper in the production of printed circuit boards.

BACKGROUND OF THE INVENTION

As is well known in the art, in the manufacture of printed electroniccircuits a laminate of copper and etch resistant material, usuallyplastic, is used. A common method of obtaining the circuits is to maskthe desired pattern on the copper surface of the laminate with aprotective resist material, which is impervious to the action of an etchsolution. In a subsequent etching step, the unprotected areas of thecopper are etched away, while the masked areas remain intact and providethe desired circuiting supported by the plastic. The resist material canbe a plastic material, an ink or a solder.

In the last few years, the industry has more and more turned to hydrogenperoxide-sulfuric acid systems for etching the electronic circuitboards, due to the low cost of the etching solutions and to the relativeease with which copper values can be recovered from the spent etchsolutions.

However, there are many problems connected with the use of hydrogenperoxide as an ingredient in the etchants. It is a well known fact thatthe stability of hydrogen peroxide in a sulfuric acid-hydrogen peroxidesolution is detrimentally affected by the presence of heavy metal ionssuch as copper ions. Thus, as etching proceeds and copper ion content ofthe etchant thereby increases, the etch rate will experience a seriousdrop-off due to the decomposition of the hydrogen peroxide in the etchbath, which will soon be exhausted. In order to improve the capacity ofthese etchants, various stabilizers have been suggested and used withsome success for abatement of the hydrogen peroxide decomposition due tothe presence of copper ions.

Although considerable retardation of the metal ion-induced hydrogenperoxide decomposition can be achieved by the addition of a suitablestabilizer, the etch rates of the stabilized hydrogen peroxide-sulfuricacid etchants have, generally, been quite low and in need of improvementespecially at high copper ion concentrations. It has, therefore, beensuggested in the prior art to add a catalyst or promoter to improve theetch rate. Specific examples of such catalysts or promoters are theinorganic thiosulfates disclosed in U.S. Pat. No. 4,130,455, and themetal ions disclosed in the U.S. Pat. No. 3,597,290, such as silver,mercury, palladium, gold and platinum ions, which all have a loweroxidation potential than that of copper. Other examples include those ofU.S. Pat. No. 3,293,093, i.e. phenacetin, sulfathiazole and silver ion,or the various combinations of any of the above three components withdibasic acids, as disclosed in U.S. Pat. No. 3,341,384, or with thephenyl ureas or benzoic acids of U.S. Pat. No. 3,407,141, or with theurea and thiourea compounds of U.S. Pat. No. 3,668,131.

Another problem often encountered using hydrogen peroxide-sulfuric acidetchants is that etching rates are adversely effected by the presence ofeven small amounts of chloride or bromide ions, and usually ordinary tapwater cannot be used in preparing the etching solution. It is,therefore, required that these ions be removed either by deionization ofthe water or by precipitation of the contaminating ions, e.g. withsilver ions added in the form of a soluble silver salt. It has also beensuggested in the prior art (U.S. Pat. No. 4,040,863) to add up to 5g/liter of either cyclohexanol or methylcyclohexanol to the etchants asinhibitors against the detrimental effect of chloride ions.

An object of the present invention is therefore to provide a novel,highly efficient aqueous composition for the dissolution of metals.

Another object is to provide an improved method for the dissolution ofmetals, e.g. copper or alloys of copper, at high rates.

Still another object of the invention is to provide an etchingcomposition and process, which are insensitive to relatively highconcentrations of chloride and bromide ions.

Other objects of the invention will become readily apparent from thedetailed description set forth hereinafter.

THE INVENTION

In accordance with the present invention there is provided a compositionwhich comprises an aqueous solution of from about 0.2 to about 4.5 grammoles per liter of sulfuric acid, from about 0.25 to about 8 gram molesper liter of hydrogen peroxide and a catalytically effective amount ofan additive of 3-sulfopropyldithiocarbamate sodium salt having thegeneral formula ##STR2## wherein R is selected from the group consistingof C₁ -C₄ alkyl, phenyl, C₇ -C₁₀ alkaryl and C₇ -C₁₀ aralkyl.

Significantly improved metal dissolution rates are obtained when theconcentration of the catalyst is maintained at about 0.1 gram per literand higher. Preferably, the concentration should be in the range fromabout 0.5 to about 10 grams per liter, although higher values can alsobe used. There is, however, no particular added advantage in using suchexcess quantities.

The sulfuric acid concentration of the solution should be maintainedbetween about 0.2 to about 4.5 gram moles per liter and preferablybetween about 0.3 and about 4 gram moles per liter. The hydrogenperoxide concentration of the solution should broadly be in the range offrom about 0.25 to about 8 gram moles per liter and preferably limitedto 1 to about 4 gram moles per liter.

The remaining portion of the solution is made up with water which doesnot need any special pretreatment to remove free chloride and bromideions to the conventional level of 2 ppm or less. Nor is it necessary toadd any compounds to the solution in order to inhibit or precipitate thechloride and bromide contaminants otherwise harmful to the etchingprocess. It has been found that the compositions of this invention cancontain relatively large amounts of the contaminants, such as 100 ppmand even higher, without any significant deleterious effect on etchrates.

The solutions may also contain other various ingredients such as any ofthe well known stabilizers used for counteracting heavy metal ioninduced degradation of hydrogen peroxide. Examples of suitablestabilizers include those disclosed in U.S. Pat. Nos. 3,537,895;3,597,290; 3,649,194; 3,801,512 and 3,945,865. The aforementionedpatents are incorporated in this specification by reference. Of course,any of various other compounds having a stabilizing effect on acidifiedhydrogen-peroxide metal treating solutions can be used with equaladvantage.

Also, any of the additives known to prevent undercutting, i.e. side orlateral etching, can also be added, if desired. Examples of suchcompounds are the nitrogen compounds disclosed in U.S. Pat. Nos.3,597,290 and 3,773,577, both incorporated in this disclosure byreference. However, in the present invention, the use of such additivesis not necessary, because of the rapid etch rates obtained due toinclusion of the catalyst in the etching compositions.

The solutions are particularly useful in the chemical milling andetching of copper and alloys of copper, but other metals and alloys mayalso be dissolved with the solutions of this invention, e.g. iron,nickel, zinc and steel.

When using the solutions to dissolve a metal, conventional operatingconditions for the particular metal are employed. Thus, in the etchingof copper usually temperatures between about 105° to about 140° F.should be maintained and preferably the operating temperature should bebetween about 120° and about 135° F.

The solutions are eminently suited as etchants using either immersion orspray etching techniques. The etch rates obtained with the compositionsof the invention are extremely fast, e.g. etch times in the order ofabout 0.75 to 1 minute are typical when etching copper laminatescontaining 1 oz. copper per square foot. Because of these high etchrates the compositions are especially attractive as etchants in themanufacture of printed circuit boards, where it is required that arelatively large number of work pieces be processed per unit time foreconomical reasons as well as for minimizing detrimental lateral etchingor undercutting of the edges under the resist material. Anotherimportant advantage of the invention is that clean etchings areachieved.

The following examples are provided as illustration of the invention.

EXAMPLES 1-3

Etching tests were carried out in a DEA-30 spray etcher with hydrogenperoxide-sulfuric acid etchants. Copper laminates having a coating ofone ounce copper per square foot were treated at 125° F. with theetchants. The control etch solution (Example 1) contained 15 percent byvolume of 66° Baume sulfuric acid (2.7 gram moles/liter), 12 percent byvolume of 55 wt. % hydrogen peroxide (2.4 gram moles/liter) and 73percent by volume of water. In addition, the solution contained 15grams/liter of copper sulfate pentahydrate and 1 gram/liter of sodiumphenol sulfonate. The etch time, i.e. the time required to completelyetch away the copper from a board was 6 minutes and 45 seconds for thecontrol etch solution of Example 1.

Examples 2 and 3 were carried out exactly as Example 1, except to thecontrol etch solution there was added a promoter, i.e.3-sulfopropyl-N,N-dimethyldithiocarbamate sodium salt in the amountsindicated below in Table 1, which also shows the significant decrease inetch time resulting from the respective additions. For instance, at 2g/liter of promoter addition, an 8 fold increase in etch rate wasobtained.

                  TABLE 1                                                         ______________________________________                                        Example    Promoter - g/l  Etch Time                                          ______________________________________                                        1          0               6 min 45 sec                                       2          0.5             1 min 20 sec                                       3          2.0                55 sec                                          ______________________________________                                    

EXAMPLES 4-6

To demonstrate the insensitivity of the compositions of the invention tothe presence of chloride ions, the following experiments were carriedout. Copper laminates (2"×2") having a coating of one ounce copper persquare foot were immersion etched in mildly agitated solutions (250 ml)maintained at 125° F. Each of the solutions contained 20 volume percent66° Baume sulfuric acid, 10 volume percent (50% w/w) hydrogen peroxide,70 volume percent deionized water and 1 gram/liter of3-sulfopropyl-N,N-dimethyldithiocarbamate sodium salt. The solutionswere stabilized with 1.0 grams/liter sodium phenolsulfonate andcontained 2 g/l of copper added as copper pentasulfate. To the solutionof Example 5 there were further added 20 ppm of chloride ion, while 100ppm of chloride ion were added to the solution of Example 6. The etchtime was established in each of the experiments, the pertinent data ofwhich are shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Example   Added Cl.sup.-, ppm                                                                           Etch Time                                           ______________________________________                                        4         0               6 min 23 sec                                        5         20              6 min 58 sec                                        6         100             7 min  5 sec                                        ______________________________________                                    

It should be noted that consistently superior results are obtained withthe solutions of this invention in large scale operations e.g. by sprayetching techniques. Specifically, the increase in etch rate as comparedto that of a control solution is much more pronounced and also theactual etch times are substantially lower than those obtained using thesmall scale immersion technique described above.

It is obvious to those skilled in the art that many variations andmodifications can be made to the specific embodiments discussed above.All such departures from the foregoing specification are consideredwithin the scope of this invention as defined by this specification andthe appended claims.

What is claimed is:
 1. A method of metal dissolution which comprisescontacting a metal with an aqueous solution containing from about 0.2 toabout 4.5 gram moles per liter of sulfuric acid, from about 0.25 toabout 8 gram moles per liter of hydrogen peroxide and a catalyticallyeffective amount of an additive of 3-sulfopropyldithiocarbamate sodiumsalt having the general formula ##STR3## wherein R is selected from thegroup consisting of C₁ -C₄ alkyl, phenyl, C₇ -C₁₀ alkaryl and C₇ -C₁₀aralkyl.
 2. The method of claim 1, wherein said additive is provided ata concentration of at least about 0.1 gram per liter.
 3. The method ofclaim 1, wherein said additive is provided at a concentration in therange from about 0.5 to about 10 grams per liter.
 4. The method of claim1, wherein the aqueous solution contains sodium phenolsulfonate as astabilizer to reduce the degrading effect of heavy metal ions onhydrogen peroxide.
 5. The method of claim 1, wherein the hydrogenperoxide concentration is maintained between about 1 and about 4 grammoles per liter.
 6. The method of claim 1, wherein the sulfuric acidconcentration is maintained between about 0.3 and about 4 gram moles perliter.
 7. The method of claim 1, wherein the metal is copper or an alloyof copper.
 8. The method of claim 1, wherein the dissolution is carriedout in the presence of free chloride or bromide ions in excess of 2 ppm.9. The method of claim 1, wherein R is a C₁ -C₄ alkyl group.
 10. Themethod of claim 1 wherein the additive is3-sulfopropyl-N,N-dimethyldithiocarbamate sodium salt.
 11. A compositionfor metal dissolution comprising an aqueous solution of from about 2.0to about 4.5 gram moles per liter of sulfuric acid, from about 0.25 toabout 8 gram moles per liter of hydrogen peroxide and a catalyticallyeffective amount of an additive of 3-sulfopropyldithiocarbamate sodiumsalt having the general formula ##STR4## wherein R is selected from thegroup consisting of C₁ -C₄ alkyl, phenyl, C₇ -C₁₀ alkaryl and C₇ -C₁₀aralkyl.
 12. The composition of claim 11, wherein the additive isprovided at a concentration of at least about 0.1 gram per liter. 13.The composition of claim 11, wherein the additive is provided at aconcentration in the range from about 0.5 to about 10 grams per liter.14. The composition of claim 11, additionally containing sodiumphenolsulfonate as a stabilizer for reducing the degrading effect ofheavy metal ions on hydrogen peroxide.
 15. The composition of claim 11,wherein the hydrogen peroxide concentration is maintained between about1 and about 4 gram moles per liter.
 16. The composition of claim 11,wherein the sulfuric acid concentration is maintained between about 0.3and about 4 gram moles per liter.
 17. The composition of claim 11,containing more than 2 ppm of free chloride or bromide ions.
 18. Thecomposition of claim 11, wherein R is a C₁ -C₄ alkyl group.
 19. Thecomposition of claim 11, wherein the additive is3-sulfopropyl-N,N-dimethyldithiocarbamate sodium salt.